Bip39 Checksum. The seed phrase is often converted into a variety that's used as the seed integer to the deterministic wallet that generates all the key pairs used in the wallet. For words that only have 3 letters, there isn't a 4th letter. One example is Using the word “add”, https://problogdirectory.com/listings13077156/bip39-seed-https-linktr-ee-bip39-not-known-details-about-bip39-online